1. Field of the Invention
The invention relates to the field of display and test equipment for video monitoring, production and the like, and in particular provides a method and apparatus whereby pixel display characteristics are illustrated in a manner that indicates compliance or violation of color gamut limitations applicable to the color space encoding technique being employed.
2. Prior Art
In color video display, a picture field is defined from an array of discrete picture elements. Each individual picture element or pixel comprises three primary-color portions, such as dots or short lines that are closely spaced. A cathode ray tube display, for example, has trios of red, blue and green phosphors. The current amplitude of an electron beam applied to the phosphors causes the red, blue and green dots at each pixel element to emit red, blue and/or green light, respectively. The specific proportion of light emission in the primary colors defines a full spectrum as well as black and white, and the colors of all the pixels in the field make up the picture display.
The signals that drive the light emissions in the primary colors can be encoded in a number of ways. The RGB encoding method provides separate red, blue and green signals, the respective amplitudes of which encode luminance, saturation and hue. Composite encoding uses luminance and color difference signals to encode the same variables of luminance, saturation and hue.
The signal characteristics that encode the different variables are obviously critical to the appearance of the picture. Video production and test equipment typically includes test pattern generators for exercising the video equipment in a predetermined way, and also test equipment such as a display, in which the color characteristics of the signal are illustrated. Such equipment usually includes a vectorscope for displaying information about the color portion of the video signal at any particular time, specifically, hue and saturation. The vector display is a two dimensional polar graph wherein the hue is represented by the angle of the displayed coordinate from the positive X axis, and the saturation is represented by the radius or distance from the origin. The color characteristics of each pixel correspond to a point on the vectorscope display.
Although the vector display is polar wherein angle defines hue and radius defines saturation, the vector display also can be considered simply an XY plot of two color difference components of the video signal. The X and Y ordinates or axes encode I versus Q for NTSC; U versus V for PAL, R-Y versus B-Y for analog component, and so on. The typical test pattern has several color bands wherein all the pixels in the band have given characteristics, and appears as a corresponding number of bright dots at different points in the vector display. A regular video program most often contains many varied combinations of color characteristics that change constantly and appear on the vector display as a changing indistinct shape.
There are three variables that encode the characteristics of a pixel in a display, such as red, blue and green amplitude in RGB, or luminance and two color difference values, for analog components. The three variables are sometimes referred to as a color space and the different encoding techniques are termed different color spaces. The characteristics of each pixel have particular values of three variables. If the variables are plotted as orthogonal axes, those pixel characteristics locate a point in a volume, hence a color space. The volume might be considered a rectilinear volume with mutually perpendicular sides, each of which extends from a minimum value to a maximum value of each of the three variables according to the color space being used. However, the definition of color is made more complex by a number of factors. There are differences in human perception of the respective primary colors whereby different ranges may be appropriate for different values. The three values in color space also are interrelated in some color space definitions, such as color difference definitions which are based on the difference or comparative levels of two colors. As a result of these and other factors, the allowable volume or universe of permissible points in a color space turns out to be an irregular volume within a rectilinear volume circumscribed by maximum and minimum values for each of the three variables in the color space.
To illustrate some of the potential complexity, in component digital video, the color space may be based on a fundamental luminance equation:
Ey=0.299Er+0.587Eg+0.114Eb,
which gives the luminance Ey in terms of the three primaries Er, Eg, and Eb. For the CCIR-601 standard, the two color components which are digitized are:
Cr=KCr*(Erxe2x88x92Ey),
and
Cb=KCb*(Ebxe2x88x92Ey).
They are given by:
Erxe2x88x92Ey=0.701Rxe2x88x920.587Gxe2x88x920.114B
Ebxe2x88x92Ey=xe2x88x920.299Rxe2x88x920.587G+0.886B
Luminance Y has a permitted range from zero to one. Differences Erxe2x88x92Ey and Ebxe2x88x92Ey have ranges +0.701 to xe2x88x920.701 and +0.886 to xe2x88x920.886. They are renormalized by applying coefficients:
KCr=0.5/0.701=0.713
and
KCb=0.5/0.886=0.564
which gives re-normalized color differences
ECr=0.500Rxe2x88x920.419Gxe2x88x920.081B
ECb=xe2x88x920.169Rxe2x88x920.331G+0.500B.
Luminance Y often is quantized or digitized to 220 levels, that is, zero to 220, with xe2x80x9cblackxe2x80x9d being at level 16 (i.e., luminance levels below 16 are blacker-than-black). The decimal value of Y prior to quantization is
Y=219(Ey)+16
where Ey is the 0-1 range continuous version, and Y is either the nearest integer (8-bit version) or the fractional version with two bits of fractional value maintained (10-bit version). Similarly, the color difference components are quantized to 225 levels with an offset or zero level equal to 128 which gives:
Cr=160(Erxe2x88x92Ey)+128
Cb=126(Ebxe2x88x92Ey)+128.
Often, the RGB components have the values 0-255, and the conversions used are:
Y=0.257R+0.504G+0.098B+16
Cr=0.439Rxe2x88x920.368Gxe2x88x920.071B+128
Cb=xe2x88x920.148Rxe2x88x920.291G+0.439B+128.
In a video device or in a digital processor or other situation, it may be convenient for various reasons to employ a certain one of the encoding techniques for some purposes and a different encoding technique for other purposes. Thus in a color television receiver, for example, video is received and decoded from a composite signal. The receiver processes or converts the color information to separate R, G and B signals to modulate the electron beam current of three separate electron beam guns positioned to excite phosphor dots of the respective color.
A constant problem in video data processing, recordation and replay is encountered because there is a disparity between the allowable ranges of different component and composite color spaces. Combinations of values that are well within the allowed range of a color difference component video system, for example, may produce signal amplitudes that are well outside of the allowable ranges when the signal is transcoded or converted into its equivalent values in composite or RGB color space.
Excursions beyond the permissible bounds of color definition in one or another component or composite color definitions or color spaces are practically impossible to quantify by observing the signals on a waveform monitor. It is possible to envision a waveform monitor that processes video data by transcoding, for example, from color difference video format to RGB format, and then plots the resulting RGB values in a manner that shows when one or more of the transcoded R, G and B values goes out of permissible range. The operator (or other means) then can view or otherwise monitor the excursions directly, and can determine when the R, G and/or B values have gone out of range. Such a monitor still does not show directly which color(s) or component(s) of the source are causing the illegal excursion. Viewing the excursions that occur when a color difference format is encoded to a composite format is not possible. Yet these measurements are critical if a signal is to be correctly or legally encoded.
Specialty displays have been attempted. Tektronix, Inc. has proposed a display intended to assist in viewing relationships between color difference color space and RGB and composite color spaces. In one arrangement, shown for example in U.S. Pat. No. 5,307,087, a double diamond shaped display field is provided to represent the allowable or legal RGB limits (or xe2x80x9cgamutxe2x80x9d) of a color difference signal. An arrowhead display to show composite excursions of a color difference signal is proposed in U.S. Pat. No. 5,519,440. U.S. Pat. No. 5,311,295 discloses a test device with a transcoder and gamut error detector and discusses comparators for generating a gamut error alarm. The disclosures of these references are hereby incorporated for their teachings regarding gamut errors and error detection.
It is a helpful diagnostic tool to know that a gamut error is occurring, namely that an encoded or transcoded signal is going out of limits. However, it would be even more helpful if a test apparatus could not only display excursions and indicate somehow (preferably visually) that legal limits are being exceeded, but also which carries information that is useful for identifying why the error is occurring.
It is an object of the invention to provide a display method and apparatus for conveniently and directly identifying the occurrence of a color gamut error in a signal, particularly upon transcoding of the signal between definitions or color spaces, and also providing information that associates a gamut error in one color space with variables in a different color space that can account for such error.
It is also an object of the invention to provide two forms of display of color characteristics of one or more pixels, one of the forms of display representing the color characteristics in a first color space and the other representing the characteristic in a second color space, wherein each of the first and second forms of display for one of said first and second color spaces carries information that represents the characteristics of said one or more pixels in the other of said first and second color spaces, whereby the displays assist in identifying the source of color gamut errors occurring in at least one of such color spaces.
These and other objects are accomplished by characterizing a sampled video input signal in two different color space definitions, such as a color-difference color space and a primary-color color space. Certain values that are legal in one of the color spaces, for example the color difference space, can produce primary color amplitudes that are out of legal color gamut range. Such gamut violations are indicated according to the invention using a polar plot in which phase angle corresponds to hue as in a vectorscope. The color difference and/or primary color values in either or both color spaces are depicted as ranges of radii in the polar plot, which are compared to circular limit lines that can represent the legal gamut limits. In color difference color space, the signal excursion is plotted as a function of hue phase angle, producing radially inner and outer values in a plot. The excursion is calculated or processed as luminance plus and minus saturation as a function of hue phase angle. In primary color color space, the amplitudes or excursions of the three primary color components are plotted as separate radii as a function of hue phase angle, also producing radially inner and outer values (except at black and white, where the component amplitudes are equally at the maximum or minimum). The plot or the display is provided with concentric lines showing the maximum and minimum legal gamut values, the minimums preferably shown with an offset whereby values below a minimum or negative values are plotted inside an inner circle but at the correct phase angle. By this plotting as a function of hue in two alternative or simultaneous color space plots, the display is useful for assisting in identifying the underlying reason for gamut violations which may occur. This processing and display technique is readily useful to persons familiar with vectorscope displays, particularly if the information is plotted by the same hue phase angle, providing a useful visible measure of the characteristics of a video signal.
A number of other objects and attributes of the invention will be made apparent by the following discussion of certain practical embodiments and examples.